Conventionally, with regards to a flow rate measuring device in which ultrasound is used, there is known a flow rate meter, referred to as a propagation velocity difference-type ultrasonic flow rate meter, for propagating ultrasound into a fluid flowing through a tube path, obtaining the velocity of the fluid from the difference between the velocity of ultrasound propagation from upstream to downstream and the velocity of ultrasound propagation from downstream to upstream of the flow, and measuring the flow rate of the fluid flowing through the tube path on the basis of the velocity of the fluid.
However, with regards to the above-mentioned flow rate measuring device in which ultrasound is used, waves propagating through the pipe have not been properly theoretically analyzed, and optimization has therefore been difficult.
The inventors of the present invention have been conducting research on the relationship between the flow velocity of a fluid flowing in a pipe and the propagation velocity of guided waves excited by an ultrasound transmission element; and discovered, as a result, that a change in the flow velocity of the fluid affects the propagation velocity of the guided waves propagating through the pipe. The inventors are presently continuing the research.
In the present specification, “guided waves” refers to ultrasound that propagates in a longitudinal direction along a plate, rod, pipe, or a similar medium that has a boundary.
Also, the applicants of the present specifications have, in the past, filed Japanese Patent Application No. 2006-109218 (see JP-A 2007-298275; hereafter referred to as “Prior Art 1”) and Japanese Patent Application No. 2007-280888 (see JP-A 2009-109299; hereafter referred to as “Prior Art 2”) in relation to inventions of a flow rate measuring device in which guided waves are used.
In Prior Art 1, it is verified that a wave propagating through a pipe filled with static water is a guided wave; and there is proposed a flow rate measuring device in which: two ultrasound oscillators are provided, with a distance L present therebetween, on an outer surface of a pipe for channeling a fluid; one of the two ultrasound oscillators is actuated as a transmission element, and the other is actuated as a reception element, with respect to each other; and there is provided a control/analysis device for obtaining the flow velocity of the fluid from the propagation time difference between a propagation time T1 and a propagation time T2, the propagation time T1 being a time in which guided waves, excited by being driven by the ultrasound oscillator with the pipe and the internal fluid acting as a single medium, propagate from upstream to downstream between the ultrasound transmission and reception elements that are separated by a spacing of L, and the propagation time T2 being a time in which the guided waves propagate from downstream to upstream.
In Prior Art 2, there is proposed a flow rate measuring device which is a modification of the flow rate measuring device according to Prior Art 1, the flow rate measuring device being one in which ultrasound is used, in which: two ultrasound oscillators are provided, with a distance L present therebetween, on an outer surface of a pipe for channeling a fluid; one of the two ultrasound oscillators is actuated as a transmission element, and the other is actuated as a reception element, with respect to each other; and there is provided a control/analysis device for obtaining the flow velocity of the fluid from the propagation time difference between a propagation time T1 and a propagation time T2, the propagation time T1 being a time in which guided waves, excited by being driven by the ultrasound oscillator with the pipe and the internal fluid acting as a single medium, propagate from upstream to downstream between the ultrasound transmission and reception elements that are separated by a spacing of L, and the propagation time T2 being a time in which the guided waves propagate from downstream to upstream; wherein the flow rate measuring device in which ultrasound is used is characterized in that the guided waves used have a mode such that the variation in the phase velocity decreases near the longitudinal sound velocity of the fluid.
Non-patent References 1 and 2 are known analyses of guided waves propagating through a pipe filled with static water.